This invention relates to the logging of a borehole to distinguish true basement from dikes and sills encountered in the drilling of the borehole.
Sills and dikes are a category of plutons that are occasionally encountered while drilling a borehole through the earth. While the bit is in the pluton it is difficult, time consuming and expensive to distinguish between this geologic feature and true basement. Current methods used to differentiate between a sill or dike and basement include radiometric dating of the intrusive body to compare its age against Cambrian or Pre-Cambrian basement, and comparison of the intrusion's mineralogy with that of basement if the latter is known. If true basement has been encountered, of course, the drilling must be discontinued.
On the other hand, if a dike or sill has been encountered it may be possible to drill through the formation. If the dense rock of this intrusion is part of a dike or sill, it is important that drilling be continued through the intrusion. Such dense intrusions form a cap for the sedimentary rocks below and hydrocarbons are often found below these caps.
Since the drilling of deep boreholes is an extremely expensive operation, it is important that the decision as to whether or not to continue drilling be based upon the best available information. Logging tools have been successfully used to provide good information about the formations surrounding the borehole. Gamma ray, sonic, and neutron logs, for example, have been obtained from boreholes being drilled and these logs are very useful in determining whether or not drilling should be continued. These logs are not very useful in determining whether the drilling has encountered true basement. The reason is that these logging tools have a radius of investigation which is no more than a few feet surrounding the borehole. These logs provide an excellent determination of, for example, density of the formations immediately surrounding the borehole. However, the density surrounding the borehole will be the same whether true basement, or an intrusion of the same density has been encountered.
Recently, borehole gravimeter logging has been developed. Reliable borehole gravimetric techniques provide an indication of the bulk density of rock 50 feet or more from the bore of the hole being logged. While gravimetric logs indicate formation bulk density at great distances from the borehole, they frequently are not true indicators of whether true basement or a dense intrusion has been encountered while drilling. The reason is that ambiguities in gravimetric log interpretation sometimes obscure sills or dikes from true basement.
It is an object of the present invention to provide a logging technique which reliably distinguishes true basement from a sill or dike encountered during the drilling of a borehole.